THE STET7GGLE EOE EXISTENCE. 199 



240. Origin of Weeds.^ — By far the larger proportion of 

 our weeds are not native to this country. Some have been 

 brought from South America and from Asia, but most of the 

 introduced kinds come from Europe. The importation of 

 various kinds of grain and of garden-seeds mixed with seeds 

 of European weeds will account for the presence of many of 

 the latter among us. Others have been brought over in the 

 ballast of vessels. Once landed, European weeds have suc- 

 ceeded in establishing themselves in so many cases because 

 they were superior in vitality and in their power of repro- 

 duction to our native plants. This may not improbably be 

 due to the fact that the vegetation of Europe and the neigh- 

 boring portions of Asia, much of it consisting from very early 

 times of plants of comparatively treeless plains, has for ages 

 been habituated to grow in cultivated ground and to :contend 

 with the crops which are tilled there. 



241. Plant Life maintained under Difficulties. — Plants 

 usually have to encounter many obstacles to their growth or 

 even to their bare existence. Eor every plant which succeeds 

 in reaching maturity and producing a crop of spores or of 

 seeds, there are hundreds or thousands of failures. It is 

 easy to show by calculation in the case of any particular 

 kind of plant, how small a proportion the seeds which live 

 must bear to those which are destroyed. The common 

 morning-glory (Ipomoea purpurea) is only a moderately pro- 

 lific plant, producing, in an ordinary soil, somewhat more 

 than 3,000 seeds. ^ If all these seeds were planted and grew, 

 there would, of course, be 3,000 plants the second summer, 

 sprung from the single parent-plant. Suppose each of these 

 plants to bear as the parent did, and so on. Then there 

 would be : 



1 See tlie article " Pertinacity and Predominance of Weeds," in Scientific Papers 

 of Asa Gray, selected by C. S. Sargent, vol. II, pp. 234-242. 

 3 Bather more than 3200 by actual count and estimation. 



